HP Fortran Programmer's Guide (B3908-90031; September 2011)

Controlling data storage
Automatic and static variables
Chapter 3104
Automatic and static variables
By default, HP Fortran allocates stack storage for program variables. Such variables are called automatic
variables because they are allocated at each invocation of the program unit in which they are declared.
Static variables are allocated storage from static memory when the program is first loaded into memory.
They remain allocated for the life of the program.
HP Fortran allocates static storage for the following variables:
• Variables specified in a COMMON or EQUIVALENCE statement.
• Variables initialized in a type declaration statement or in a DATA statement.
• Variables specified in a SAVE or STATIC statement. A SAVE statement without a variable list specifies
static storage for all variables in the scoping unit.
• Variables in program files that have been compiled with the +save or +Oinitcheck command-line
option. See “Uninitialized variables” on page 241 for information about using these options when
porting.
Static variables have two characteristics that are of special interest:
• They are set to 0 or null value at load-time.
• They do not require re-initialization at each invocation of their program unit.
Static variables have several disadvantages. In Fortran programs that use recursion, static variables can
defeat one purpose of recursion—to provide a fresh set of local variables at each recursive call. Also, the
widespread use of static variables in a program can slow its performance: static variables are ineligible for
such fundamental optimizations as register allocation, and they can limit the optimization of program units
that use them.
The following example program illustrates the difference between automatic and static variables. The
program consists of a main program unit that calls a recursive internal subroutine. The subroutine
increments two variables (stat_val and auto_val), prints the updated variables, and then calls itself
recursively. Neither of the two variables is explicitly initialized, but stat_val is declared with the SAVE
attribute, which means that it is allocated static storage and is pre-initialized to 0 by the compiler.
The program is shown below.
Example 3-1 recursive.f90
PROGRAM main
! This program calls a recursive internal subroutine.